"The increase we see is most likely due to better ascertainment," said Stephen Blumberg, lead author of the study and a senior scientist at the CDC's National Center for Health Statistics.

The analysis, released Wednesday, found large numbers of children who were diagnosed with an autism spectrum disorder years after their symptoms probably became apparent — especially older boys whose parents gauged their cases as mild.

The wide-ranging disorder emerges by age 3 and is diagnosed by its collection of symptoms, including problems with socialization and communication as well as repetitive behaviors or obsessive interests.

The number of children diagnosed with autism has been climbing for more than two decades, though it is unclear to scientists whether children today are at greater biological risk than those in past generations. Much of the rise can be explained by a broadened definition of the condition, rising awareness of it and the fact that a formal diagnosis can lead to better access to services, experts say.

The most comprehensive assessment of autism prevalence in the U.S. comes from a surveillance network set up by the CDC in the 1990s. It periodically scours school and health records of 8-year-olds in several states, looking for autism diagnoses and clusters of symptoms.

Its most recent analysis, using 2008 data, found an overall rate of 1 in 88, up from 1 in 150 in 2000.

Tremendous geographic and racial variation, however, suggested that social factors played an important role in determining whether a child was diagnosed with the condition.

The new study is based on the National Survey of Children's Health, a survey of more than 90,000 households with children that is conducted every four years.

Parents were asked whether their children had been diagnosed with autism, when they where diagnosed and whether the cases were mild, moderate or severe.

The diagnosis rate for boys ages 6 to 17 was 3.2%, up from 1.8% in 2007. The rate for girls was 0.7%, which is not a statistically significant change from the previous rate of 0.5%.

The rate among youths 14 to 17 more than doubled, from 0.7% to 1.8%. Nearly 14% of these received their diagnoses after 2007, though their symptoms would have been present long before. And nearly 70% of these cases diagnosed later were classified by parents as mild.

Laura Schieve, a CDC epidemiologist and coauthor of the study, said many milder cases were not formally identified until children entered school and it became clear that they required extra services. Sometimes the full extent of a child's problems is not seen until later grades, as school and social life become more challenging.

An autism diagnosis can open the door to behavioral therapy or help with speech and social problems.

The study did not attempt to verify whether children whose parents identified them as autistic met the formal criteria for the disorder.

Some autism advocates said the new study should be used to highlight the growing need for autism-related services.

"I'm tired of people worrying about why this is happening," said Judy Mark of the Autism Society of Los Angeles. "We need to focus on the fact that these people are out there."